Types of OS and OS Components

Types of Operating Systems Within the broad family of operating systems, there are generally four types, categorized based on the types of computers they control and the sort of applications they support. The categories are: Real-time operating system (RTOS) - Real-time operating systems are used to control machinery, scientific instruments and industrial systems. An RTOS typically has very little user-interface capability, and no end-user utilities, since the system will be a "sealed box" when delivered for use. A very important part of an RTOS is managing the resources of the computer so that a particular operation executes in precisely the same amount of time, every time it occurs. In a complex machine, having a part move more quickly just because system resources are available may be just as catastrophic as having it not move at all because the system is busy. Single-user, single task - As the name implies, this operating system is designed to manage the computer so that one user can effectively do one thing at a time. The Palm OS for Palm handheld computers is a good example of a modern single-user, single-task operating system.

Types of Operating Systems Single-user, multi-tasking - This is the type of operating system most people use on their desktop and laptop computers today. Microsoft's Windows and Apple's MacOS platforms are both examples of operating systems that will let a single user have several programs in operation at the same time. For example, it's entirely possible for a Windows user to be writing a note in a word processor while downloading a file from the Internet while printing the text of an e-mail message. Multi-user - A multi-user operating system allows many different users to take advantage of the computer's resources simultaneously. The operating system must make sure that the requirements of the various users are balanced, and that each of the programs they are using has sufficient and separate resources so that a problem with one user doesn't affect the entire community of users. Unix, VMS and mainframe operating systems, such as MVS, are examples of multi-user operating systems.

Types of Operating Systems Five categories Batch Interactive Real-time Hybrid Embedded Two distinguishing features Response time How data enters into the system

Types of Operating Systems (continued) Batch Systems Input relied on punched cards or tape Efficiency measured in throughput Interactive Systems Faster turnaround than batch systems Slower than real-time systems Introduced to provide fast turnaround when debugging programs Time-sharing software developed for operating system

Types of Operating Systems (continued) Real-time systems Reliability is key Fast and time limit sensitive Used in time-critical environments Space flights, airport traffic control, high-speed aircraft Industrial processes Sophisticated medical equipment Distribution of electricity Telephone switching Must be 100% responsive, 100% of the time

Types of Operating Systems (continued) Hybrid systems Combination of batch and interactive Accept and run batch programs in the background Interactive load is light Embedded systems Computers placed inside other products Adds features and capabilities Operating system requirements Perform specific set of programs Not interchangeable among systems Small kernel and flexible function capabilities

OS Structure To understand an OS, let’s first look at its components and then how they’re composed or organized. We’ll come back and look at each of these in detail as the course progresses. Realize that it’s never as simple as it looks. These basic concepts exist in some form in all systems, however each system implements them in a slightly different way. Also, the divisions between components may not be as clean in the real world as in the model

Process Management An operating system executes many kinds of activities: user programs batch jobs or command scripts system programs: print spoolers, name servers, file servers, network listeners, etc... Each of these “execution entities” is encapsulated in a process. The process includes the execution context (pc, registers, vm, resources, etc) and all info the activity (program) needs to run. The OS schedules processes to run.

Processes A program is a passive thing -- just a file on the disk with code that is potentially runnable. A process is one instance of a program in execution; at any instance, there may be many processes running copies of a single program (e.g., an editor): each is a separate, independent process. Process A Process B Code Stack PC Registers Code Stack PC Registers page tables resource counts page tables resource counts .... .... 4

Process Operations Processes are fundamental OS-provided objects. The OS supports operations on processes, e.g.: create a process delete a process suspend a process resume a process inter-process communication inter-process synchronization create/delete a subprocess

Memory Management Primary memory is the direct access storage for CPU. Programs must be stored in memory to execute. OS must: allocate memory space for programs (both explicitly and implicitly) deallocate memory space when needed maintain the mappings from virtual to physical memory (page tables) decide how much memory to allocate to each process, and when a process should be removed from memory (policies)

I/O Management Much of the OS kernel is concerned with I/O. The OS provides a standard interface between programs (user or sytem) and devices. Device drivers are the processes responsible for each device type. A driver encapsulates device-specific knowledge, e.g., for device initiation and control, interrupt handling, and errors. There may be a process for each device, or even for each I/O request, depending on the particular OS.

Secondary Storage Management Secondary storage (disk) is the persistent memory, i.e., it endures system failures (we hope). Low-level OS routines are typically responsible for low-level disk function, such as scheduling of disk operations, head movement, error handling, etc. These routines may also be responsible for managing space on the disk.... BUT, the line between this and the file system is very fuzzy...space management functions may belong in the file system.

File Management Secondary storage devices are too crude to use directly for long-term storage. The file system provides logical objects and logical operations on those objects. A file is the basic long-term storage entity: a file is a named collection of persistent information that can be read or written. The file system supports directories -- special files that contain names of other files and associated file information.

File Management File system provides standard file operations, e.g.: file creation and deletion directory creation and deletion manipulation of files and directories: read, write, extend, rename, protect.... file copy The file system also provides general services, e.g.: backup maintaining mapping information accounting and quotas

Protection System protection is a general mechanism throughout the OS all resources objects need protection memory processes files devices protection mechanisms help to detect errors as well as to prevent malicious destruction

Command Interpreter process that handles interpretation of user input commands from keyboard (or script files) on some systems, command interpreter is a standard part of the OS on others, it’s simply a non-privileged process that interfaces to the user, permitting replacement of interpreter with others on others, there’s not really a command language (e.g., the MacIntosh has no commands in the conventional sense) A kernel is the part of the operating system that mediates access to system resources. It's responsible for enabling multiple applications to effectively share the hardware by controlling access to CPU, memory, disk I/O, and networking.